I started biking my 15 mile each way commute around a month ago and still haven't quite worked up to doing it 4 or 5 days a week. I have been doing between 60-90 mi a week, or 4-6 times one way or the other. I started on my $40 schwinn collegiate 3 spd and it was fine, very comfortable, but a little slow and worthless on the hills or with any sort of headwind. I picked up a mid-90's Chinese schwinn mtb at a yard sale last fall with a bent front wheel for $8 and used a donor mtb my son outgrew to get it on the road and that's what I'm using now. I would do the trip in about 1 hr 15 on the collegiate and I do it right at about an hour on the mtb. More gears, lighter, larger frame so I'm a bit more spread out and "aero" than sitting bolt upright. You can def go cheap if you keep your eyes out for deals. No big problems on either bike of mine yet. See how they look at the end of summer I guess. I enjoy the ride a little more every time I do it, I hope you have a good experience as well.

*edit- I am not remotely an athlete, I am an overweight smoker in my mid-thirties who started riding casually last year and loved it. Also have some muscles missing from my left thigh from surgery a couple years ago. I tell you this because if I can do it, you probably also can.