Just wondering how far off I am ...

I am not a coach.,,,,, that doesn't stop me giving advice.

Going from 8 miles to 26.2 miles in 8 weeks is a recipe for injury, IMHO.  Could you complete a mary?  probably.  painfully.  

Is the HIM your A race?  If so, why jeopardize it by doing something you aren't trained up for?  BTW, many coaches recommend training for a full mary but doing a half mary during a season.

^^^ X2. Put the mary in your plans for next year. 

I'm no coach either, so I referenced a Hal Higdon marathon plan and that plan has a 'novice' runner doing a 15 mile run 8 weeks out from the marathon.  You are at half that.

I would suggest finishing the half feeling really strong and putting up a good number, than risk injury or just trying to survive a full marathon.

Just my 0.02.

If it was me, I would not try the Full Mary by only running 20 miles a week. That is roughly 4 miles a day if you take 2 days off.

I imagine you could make it, but I am not sure it would be the best thing to attempt IMO.

ChrisM said it, recipe for injury.  Most of your runs right now are on a dreadmill.  Personally I'd go with the HM and work on increasing the weekly long runs outdoors.

Remember the goal, it's the HIM in July and the run training in the next 8 or so weeks for a HM will do wonders for it.

Long way off.  You don't want to run a marathon without having hit about three runs in the 17-20 range and if 8 is your long it will take some doing (not that it can't be done).  There is a big difference between 8 and 17 (not really linear w.r.t. what it does to your body).  The real problem with a marathon, if you try to do run it hard, is the taper before and the recovery after.  It puts a huge hole in your HIM training.  I have run 5 marathons before, but have considered a Spring marathon and rejected the thought for that reason. 

Half marathons are awesome, they don't take so much out of you, taper is minimal.  Furthermore, the aerobic benefit you get for going that hard for that distance really pays dividends w.r.t. pushing the speed at which you can run at LT faster.

Definite no-go on the marathon. For sure.

Half-marathon is still a go, and I'd recommend it.

If you still have the crazy desire/idea to go all-out for the full marathon, go do one of your required 20 milers (NOT recommended!) this upcoming weekend, and you'll feel the hurt and suffering that you'll be facing on race day. Take that TIMES TWO (no exaggeration here), and you'll have a realistic picture of the minimum pain & suffering you'll be facing on race day due to your low-mileage training.

Running miles are somewhat different than cycling miles, due to the beat-down on the legs and joints. There is a world of difference between 8 miles, 15 miles, 20 miles, 22 miles, 24, and 26 miles. Yes, at those 20+ mile distance, those extra 2 miles will feel like an extra 10 miles each, since you will be past your body's intrinsic glycogen stores, even with GUs and fuel en route. 

This is seen a lot on HS forums when low-mileage HS track "stars" who run 15-16 minute 5ks routinely post that they can run "all day" at 8-9 min/mile. Someone usually challenges them to actually run a marathon at that pace, and when they try it, if they somehow manage to finish it, they suffer a world of hurt and swear to never do something that dumb again.