djrachman, sounds pretty similar to what I have going on.  Like you I started out where I could barely lift 2 plates on the hamstring curl machine single legged.  I’m now up to 10 plates singled legged 3×5, trying to build strength with high weight, low reps.  It does seem to help, but only for a day or two.  In general tightening up my posterior chain helps but isn’t a lasting fix, maybe my quads are still dominating my hamstrings?  Maybe I need to focus on my back?  I’ve found that just staying active helps me, it doesn’t allow my hip flexors to tighten up. When I go out for a big effort on the bike or ski touring and sit around after everything tightens up.  If I stay active and do some mobility work I am much better off.  Maybe that’s why walking is helping you?  I don’t think I walk enough, I do other sports instead.  Walking seems like such a fundamental movement, it’s probably not a bad idea to increase my mileage.