Four years ago I was an avid hiker and backpacker. 18 mile dayhikes with 5,000 feet of climbing (not technical) were the norm, and a 50 lb. backpack was acceptable. I considered myself to be in good condition. . .
Untilt the day an old 10-speed was dropped off at my house by my sister. I arrogantly took it out for a 5 mile ride figuring the crossover from hiking to biking would be natural.
I pushed the bike up steep hills and hurt like crazy the next day, and the day after. I was humbled, but got back on that 31 lb. beast.
I started out with a 4.75 mi. loop, progressed to 6 mi., then to 10 and felt like I really arrived on my first 20 mi. day. It took all summer riding 2 to 4 days per week.
Be persistent and cycling will change your life. Don't forget to have fun, and don't be too hard on yourself (mentally and physically).