Hi I use use RPE as well as Zone 2.My definition of zone 2 is up to 75% of max HR or 190 -age ( I found out recently that I should revert to this calculation for the 50 + age group rather than the original MAF 180 -age). I use RPE for longer runs because of cardio drift I experience, so don't have to resort to shuffling as according to a Jack Daniels video I watched recently I should avoid as it teaches the body bad mechanics. Cheers all