Who uses FTP when running and how do you measure it?

BMusicRun

I'm used to using FTP based threshold sessions on the bike and they work really well (the bike tells me which zone I'm in and how hard I need to push it and for how long so its easy), but with running I'm struggling to work it out based on RPE (very variable, depends on mood ha) or H.R., are there any other measures you can use on the run or any sessions which stop you from cheating?