Jonnyo, you might be surprised at how similar we train (at least on the bike w/ or without a PM)... I definitely do my strongest riding alone and it's entirely based on RPE. Yes, even my intervals are done by feel. I don't sit on a number. Period. I realize a lot of people do but I don't. I just use the PM after-the-fact to reinforce/justify how hard I'm truly riding. However, even though you don't use one today, like you, I have used a PM for years and that has a strong influence on how I ride today.
Interesting enough... I often use songs to determine the length of an effort/interval...
Thanks, Chris