I think you are right, and I'm sorry you encountered this injury, but I would hate to see this thinking become an impediment to progress for cyclists. 15 miles

I think you are right, and I'm sorry you encountered this injury, but I would hate to see this thinking become an impediment to progress for cyclists. 15 miles at 10mph is just 1.5 hours of pretty leisurely exercise. 20mph is reasonable for a fit rider on a dedicated path with just an inexpensive steel-frame road bike. About your pattellar tendonitis: I developed that when I was trying for an ambitious marathon goal. It took 1.5 years to go away -- in my experience nothing helped except the time. You might want to try getting off the bike for several months and picking up something else, like swimming.