I would also say that at 70 one should be looking at keeping bike rides enjoyable rather than viewing them as detailed workout routines. Talk about ruining a pastime! Aim to just get out and ride more mainly at moderate intensity with a harder push once in a while if that's fun for you.
If you want to increase frequency long term the best way is to keep it fun and make riding part of a lifestyle rather than regimented workout routine. At your age the risk of long layoffs due to burnout (fatigue) or overuse injuries will be an issue to manage intelligently.
You can also add some resistance/strength training but again, go slow. At a recent conference (I work in health care for the elderly) a speaker talked about research that links cardio and strength training to reduced cognitive decline in older populations. Strength training was considered slightly better in that regard.
Last edited by Happy Feet; 04-23-19 at 01:49 PM.