A few days ago was my first ride on the PCs in Boulder Colorado.  My coach (Chuckie V.) and I went mostly on the bike path.  It helped me put in the time and 'coast'.  I was cheating a bit but we went for about 1hr22min and my flexors were aching after the first 30minutes.  Slow and steady.  I am glad I am back on them tough. I am just heading out again on them today - my goal: two hours with less coasting. 

One thing I noticed is that you really need to focus on your speed and cadence. You can power through them with a low cadence but once you increase it, it gets that much tougher.  I know it is going to help with my rehab in getting back into running (I broke my toe 3.5 weeks ago).  I am back running in about 1 week.    Hopefully the PCs help with the hip flexor strength and my momentum in running!  Will keep you posted!