Need bike help! (post first tri)

Just finished my first tri on Saturday! (hoping to write up a summary later)  Finished 331 / 487 overall, 16/37 on the first timers group.  Overall I was fairly pleased... except the bike.  Finished 207th overall on the swim (11:35 for 600m), and 127th overall on the run (25:30 for a 5k).  However my bike was 436th overall (43:16 for 11 miles).  Average speed was 15.3 mph.  This isn't too far off what I figured from training, and I wasn't really sure to expect.  I have to be missing something in the bike technique / training / etc, I feel like I'm in decent shape, and there's no reason that I should be in the top 30 - 40% of athlethes for the swim & run, and in the bottom 10% in the bike.

I didn't have any major issues, I didn't feel like I was horribly out of breath or out of shape for it, or that I was over-exerting myself... I just can't seem to maintain any sort of speed.  I did have an issue with a calf cramp starting out, so the first 45 seconds (after transition mat) was me massaging my calf, but once I got going it held up fine.

I started doing a lot more research this morning about it, mainly about trying to get into a higher cadence while riding, instead of more power on each revolution.  I think I've definitely  been going in too high a gear for training and the race so far.  I've got a basic bike, it's not quite a mountain bike, but the tires are nowhere near the tiny little ones.

Any basic tips to get started? I'm thinking about getting a simple bike computer, what features should I be looking for?  I saw a few for $20 at Sports Authority.

Any tips what I should change in my training? I've been doing 25-40 miles a week (3 workouts) for the past 7 weeks beforehand, building up to the end.  Should I consider a big increase in that?  Or just focus on technique and training to maintain a much higher speed over 10-12 mile workouts?  I'm a little time-constrained in when I can bike, though I am free to toss a big ride in on the weekends if need be (done one 20 mile ride so far on a weekend).  I'd like to do at least 1 more tri this year, and make a huge dent in that bike time!