I got back into cycling several years ago after a 20-year holiday. I bought an old steel-framed Pinarello at a pawn shop for $200. I aired up the crappy looking tubular tires, oiled the chain, and went for an 8 mile ride.
The next day my butt was quite sore, sore enough that I stayed off the bike for a couple days. But I got back on the bike again, and did another 8 mile ride. After riding three days a week for a couple of weeks, my butt stopped hurting.
After a month or so, I began riding 4 days a week, and I doubled my distance to 15 miles a day. On Sunday I would ride for twenty miles. I began to lose weight. After a couple months, I began to increase the speed of my rides, pushing myself hard for a few minutes, and then riding easily for a few minutes. I began to sleep batter at night, and the weight continued to come off.
After four months, I was shopping at the local bike shop, and the owner invited ms to go on their Saturday group rides. I was nervous about riding with "real" cyclists, but what the hell, I had been wearing Lycra for a few months, if I could be brave enough to be seen in public wearing bike shorts, I was brave enough to ride with a group.
There were actually two groups, the fast group for the wannabe racers (and a few real ones), and the slow group for beginners and old people. I chose to ride with the latter bunch. It didn't take long for me to become comfortable riding with others, many were far more ridiculous looking than me. I learned to ride in a pace line, and often spent so much time in the front that I burned myself out, and couldn't get back onto the end of the group.
I graduated to the fast group. But not for the reason you might think. I moved to the fast group because there was an attractive female rider whom I sometimes talked to before the rides began. It takes nerves of steel and the courage of a lion to pick up on an attractive (very attractive) woman while wearing bike shorts. Riding with real racers is nothing in comparison.
The fast ride was truly fast, but my new girlfriend was an accomplished amateur racer, and she encouraged me. The other riders in the group gave me some respect. Not due to my riding ability (I had very little) but because I had befriended the most attractive female rider out of our 100-odd person group.
In time I became quite fast. I made friends in the group, and learned about strategic riding. I began focusing on sprinting. Within one year of buying my pawnshop bike, I was finishing consistently in the top 5. My body was lean and hard, my resting pulse was in the 40's.
I bought a new bike, and entered the local amateur races. My body was hooked on endorphins, at the end of a 100 mile ride I would already be looking forward to riding the next day. I would wake up in the morning after a race with my legs so sore that I could barely walk. I would get dressed, hobble to my bike, and join the 40 mile morning group ride. After 20 minutes my legs would loosen up, and I would be hammering away at the front.
Be careful when you start riding, it can be a life-changing experience. But it's a good one.