I got into cycling a couple of months ago and did my longest ride as of date: an 80 miler. But, it took me forever - 6 hours to be exact. My issue became that later on in the ride, my legs would just jello really quicky just from pedaling at 17-20mph and I would have to take a quick minute break before I could pedal normally again. Is it just muscle fatigue or is there anything I can do to help it?