i run around 50mpw and 80% mhr (150 bpm) would have me at steady running pace, not quite reaching notional marathon pace. I'd say my 5k/10k and marathon times match the usual pace predictor calculations.
i tend to do recovery runs at 130bpm and tempo work at 165
In terms of effort level it sounds like your doc is letting you push past easy jogging pace but not letting you go fast enough that breathing becomes more strained. You could train for a relaxed marathon or possibly longer with that- racing marathons or tearing the arse of a 5k seems to be off the agenda.
having started out with HADD that looks like a great system for you. There is also a swiss doctor who recently put something out there- experimenting on himself and later on a control group he argues that large volume at low intensity trumps traditional marathon training.