I also suffer from arrhythmia (VT, VF and Torsades de pointes, the deadly ones) and mostly limit myself to <10 mile rides at a sedate pace. Once you accept your new normal and that there has to be a new normal you can move forwards.
The advice of not doing "lots of exercise" isn't terribly useful is it, but I'd most certainly take heed to that advice, which it sounds like you are doing.
Instead of doing 70 miles on a Saturday morning, do 10 miles a day over seven days. I like to set myself lower and slower goals and find off road cycling much better and more interesting doing so. The added bonus offroad is there are fewer people to laugh at you whilst you crawl up a hill at 3mph.
Another good one if you have the kit is to try and keep your in ride heart rate below a predetermined number, say below 110bpm. You'll be surprised how difficult that can be to put into practice and the focus it requires, particularly on the hills. Slower can be more difficult than you realise when you impose a low max heart rate. Slow is my new fast