Stationary bike vs Road cycling

Get a trainer, such as the Kurt Kinetic Road Machine.  Then sign up for Trainer Road.  You use your bike with the addition of a speed sensor and a cadence sensor.  Trainer Road will calculate "virtual power" using your speed and the type trainer you have.  TR has various training plans and individual workouts.  Using the guided plans you can make huge strides forward.

I believe TR is ~ $12/month.

The miles you cover on the gym bike are nearly meaningless. 

Almost every gym bike I've been on (a LOT) does not factor the resistance setting on the gym bike into distance covered. So someone who covers 1 mile but has it on zero resistance and can pedal 90rpm with nearly zero effort, would get the same credit for someone who covers 1 mile but has it set on 25 setting or some setting so strong that it's like climbing a 15% climb. 

So in summary, it's pretty hard to compare gym bike efforts to outdoors. But as a rough estimate, 'perceived effort' is actually not bad at all - if it feels harder outdoors, you can crank up the resistance on the gym bike so it matches or exceeds what you're feeling outdoors. 

I actually do regular training on a gym bike (gasp!) and I set the resistance fairly high, and there has been no problem whatsoever for me to continue improving and getting good results with it. 

Its the gym bike, not you or your road bike.

Not only is a gym bike near meaningless in terms of real training, you don't get any real world experience. You have no bike handling skills, no sense of resistance, no skills dealing with other bike traffic, passing, being passed, road etiquette, etc...If anything you would pose a real danger to yourself and other riders if you only train on a indoor bike and then go jump into a real world competitive setting such as a triathlon.

As an example, I ride a 27yr old 12 speed and I would consider myself a intermediate rider, to cover 18.5 miles at 18mph in about 1 hr takes some rather serious effort. 

This sounds actually pretty close to real world. Exactly why indoor cycling is fairly worthless for real world training. I would recommend finding a local bike shop and talking with them about local bike groups that you could start riding with. I used to be a lone wolf, but now really enjoy riding with a group of other folks. Again you really start picking up skills. For instance just this last Saturday on a 55 mile ride I was having issues passing a gal on a downhill. We where pretty evenly matched but I was just abit faster then her, but I had to keep backing off just to keep it safe. We where hitting speeds of 47mph and she is 68yrs old! I don't want to have to explain to the grand kids how I bumped grandma doing 47 down a mountain road! So safety, bike handling, road etiquette, etc all come from riding out doors and with groups of others. Although there are times when going solo is very rewarding too.

Anyways, thats my bit of opinion. Best wishes and have a successful training session and excellent first race!

There are several things that are different.  One of the big differences in riding outdoors is the traffic.  You slow down for stop signs or traffic lights.  Even on a path, you are slowing down for other people.  You also have hills to slow you down.  The downhill and uphill don't average out.  The uphills cost you more than you gain back on the downhill.  And lastly, you are probably working more on controlling the bike due to inexperience than maintaining the same effort you were indoors.  Check your heart rate on both.  I bet you are not working as hard yet outdoors. Even if you have a reasonably accurate power measurement from your indoor bike, I would generally expect a 15 mph indoor speed to translate to something like 12 mph outside on a flat course closed off to other traffic (i.e. race conditions.)

I would disagree with those who say the indoor bike is worthless.  It probably has a different body position and does not work on bicycle handling skills, and probably is unrealistic in estimating your speed.  So road cycling is better training for riding on the road.  That should be obvious.  But I understand issues such as safety and weather even convenience that may cause you choose to use the indoor bike a lot.   You are working mostly the same muscles so you are gaining fitness and getting your body ready for the race so that much is good.  Ride outside as much as you can but if the choice is between not riding at all and the exercise bike, by all means use the indoor bike.  You are getting a huge fitness improvement which is what you are looking for after all.  Riding 40-50 minutes at 15 mph regularly for 5 months is nothing to sneeze at from a fitness point of view.  Good job and good luck on your race.  

TW

I don't think there is any argument in the benefits of a stationary bike.  At the end of the day if you ride hard (or at x bpm/power output) it doesn't matter what it is on, you will still see benefits.  But, using a stationary bike to judge your performance on the road just won't work, which I think was the OP's concern.

I wanted to make a quick apology here for my above statements that riding a gym trainer style bike is near worthless. I think what I was trying to get across was in terms of real world riding experience. 

Yes of course you can get a good workout and build appropriate muscles/fitness. We can also do this on the treadmill for running and in the pool for swimming. The point I was trying to make was that you can spend all your time training in the gym and everything in the real world is going to be much harder due to natural laws of physics.

So it should be no surprise to the OP in the huge difference in effort she was experiencing.

My apologies for making what appeared to be a blanket statement that gym training is worthless, it has obvious benefits. 

Best Wishes

The bolded is patently false unless you're so green on the bike that you're a raw beginner in terms of road handling. 

I do 80-90% of my training indoors due to scheduling. 50% of my bike miles are done on a Lifecycle gym bike that's pretty upright not 'aero' like my TT bike. But I crank the heck out of the gym bike resistance, and pump on the treadmill so it's a HARD effort when it counts.

Outdoor riding and running feel MUCH easier for me to go fast as a result. There's nothing wrong at all with gym bikes provided you go hard on them, and nothing at all with indoor training. In fact, for the disciplined, indoor training is often superior to outdoor training, with its better structure, lack of interruptions (stoplights), and ability to make tiny incremental upticks in your paces when needed to keep the benefits coming.

I tend to win or podium in my AG at most races except the biggest national-class races, and most of my improvement occurred during indoor-heavy blocks, not outdoor-heavy blocks. 

Pro triathlete Lionel Sanders is also a great example of dominating the pro competition by training mostly indoors, well documented on his blog. He's not just some lowly pro - he's one of the very best, and he's been indoor training for most of his tri career.

This along with what Tech Geezer said.  It's pretty easy for a speed sensor to be off by a good margin if your wheel size isn't calibrated properly or if you don't have some type of auto pause turned on.  Waiting at a stop light for a couple minutes can easily drop your average speed a good chunk if the timer is still running.

A stationary bike generally simulates flat riding when it comes to calculating "distance" and I find it very hard to believe that with the same effort you can only manage 8.7 mph while riding on flat terrain outdoors.  It's possible you were averaging 14-16 mph while riding on flat ground but lost a lot of speed when approaching stop lights, turns, being cautious of traffic or pedestrians, or riding up hills that you didn't want to descend back down fast.