Enjoy! Your legs giving out is absolutely normal when you push it longer and harder than you are used to, even when in shape. For that kind of mileage, however, you should consider a road bike (save that one for the trails). MUCH more enjoyable for longer rides. I am 52 and started riding again 4 years ago (first time riding regularly in 25 years). Can't go without it now. I also just bought a trainer for the winter months and have been on it several mornings a week. I have my 1983 Nishiki hooked up to it. Looking for good weather this weekend to get outside!