From a 2 time OTQ.
I do the 20+ weekly long runs all year long. When done at a moderate pace say 6:20-30 they don't take much out of you when you've been doing 110+ MPW for years. If you've been in the 90-100 range maybe alternate between 16-18 and 20 milers each week in the base period.
As far as what a week looks like. For me, a typical week would look like:
MWF Easy 11 miles morning 5-6 easy afternoon. T/Th Fartlek or Intervals 12-13 miles morning, 5-6 easy afternoon. Sat Tempo usually 8 or 9 miles sometimes 10 miles, overall 13-14 miles, 5-6 in the afternoon. Su 20-22 miles. May skip the double 1 of the non-Sunday long run days.
I would not recommend the double be longer than 6. I think you get more benefit and recovery from a 12/6 then 10/10. I do know guys who do 10/10 but that was never for me. I think doubles in the afternoon after a hard workout in the AM are great at helping me feel better the next day than if I take the afternoon off. I would not recommend short doubles of 3-4 miles every day either. I would push it to 5-6 miles and double fewer days rather than shorter doubles every day.
I also do not recommend adding more doubles to shorten the longer daily run. Having longer of your daily easy runs 11-13 miles has a huge benefit to building the strength for running a competitive marathon. I've seen negative results from guys who lengthened the double to 6-8 miles in order to shorten the daily run to 9 or 10 miles.