you know, i made a similar mistake. I'm in relatively decent physical shape, i lift weights, had been doing some serious cardio training and things like that for about a month solid before i got my bike. Mind you, i had been working out heavily off and on for a few years and have always been strong and fit enough. Well i decided to take a pretty long ride when i first got my bike. 10 miles round trip. possibly the stupidest thing i've ever done. my back was killing me, my sit bones were annihilated for days afterwards and my thighs were on fire. on top of that, i had a sharp stabbing pain in my right knee.
i took a few days off, continued with my lifting and riding shorter distances and now i'm riding like 13miles at a time. still getting in better shape for riding faster and further. I love it.