It will depend on how much you ride your bike. It's not hard to do 10 mph on a bike on level terrain, your butt may think otherwise. I made the mistake of trying to ride 22 miles the first thing in the spring when the weather got nice on a rail trail. I did the first 11 miles out without any problem but the return trip was so pain full I thought that I would never make it. I learned my lesson and bought a trainer to stay in shape over the winter. I rode the whole summer before and never had any problems. Taking the winter off put my butt training back to zero.
Long story short you need some time in the saddle. Riding for an hour or two is a lot different then 5 hours at a time. If you can train at a faster pace for a shorter time you will be able to go longer at a lower pace. If you have a trainer or exercise bike the more you use it the easier it will be.