The most fundamentally important question of them all hasn't been asked yet........what was the build-up to 71 miles? If you have hundreds and hundreds of miles behind you, with a gradual build up from 10 mile rides to regular problem-free 50 mile rides, then, yes, you need to look at a saddle fit, your shorts, and some lubrication. If you've just cracked the bike shed door open last week, dusted off the cobwebs, done a 5 miler to check that everything works, then done your 71 miles, you got just exactly what you deserve. If you've not ridden much, then little-and-often is the formula to start with.