Not sure what the policy is here on linking to Reddit but I posted about my 1.5 mile barefoot run on gravel there at the barefootrunning sub. I posted it initially lamenting that I was slow and had to bail after 1.5 miles, wondering HTF do people do this? One of the best replies pointed out how gravel is a great teacher of form. You don't have the luxury of just plopping your foot down haphazardly or pushing off the ground however you want. You have to be really careful, slow and run with absolute flawless form. I was doing 14:00/mile, felt like I looked like someone's grandpa but when I was back on blacktop my body took all the lessons from gravel and applied them beautifully. I was also glad to hear from others that running on gravel is always hard and uncomfortable but that's why you do it. I got into minimalist and barefoot running to avoid the injuries I used to suffer through in shoes and I achieved that easily enough. Now I plan on working in more gravel runs to keep honest about my form as that's seriously unlocked the long miles for me. I'm not just forefoot striking I'm running tall and elastically and can tell I'm now benefiting from all those springy tendons just bouncing me along down the road.