Yes,finding the time to get the miles in is a struggle you can't always win. I hardly ever do double runs & do a standard 14 mile run usually four times a week,with something totalling around 10k down at the track on club night once a week,which would include my speedwork. With mara training I'll fit in one long run of 24 miles,but this gets chopped down a bit from 3 weeks before the next mara. Have managed to run 6 days a week now without feeling too fatigued,but I'd hardly call my approach scientific.
I've been running just under 4yrs,so I hope there's still enough time to build up endurance before age starts the slowing down process.