A lot of people will claim that running doubles is not necessary until you're at 70+ mpw. I trust that they know what they're talking about, but always wonder about a few things.
1) For someone who's slightly injury prone, wouldn't it be a little easier to run 6 in the morning and 3 in the evening at times, than just 9 straight?
2) Sometimes time off is a real benefit. I get that, which might contradict #1. I feel like my body heals well when I get one solid day of no running. I know it's best to run everyday for blood volume, but in the interest of staying healthy, why not sneak in a few short doubles and then take one full day off? Maybe do a couple of 30 min walks on the off day, but a double or two could give the benefit of allowing a rest day.
3) Does it matter on event? That is, if I'm training for things that aren't at the M/HM level, could doubling (and it's HGH benefit) be useful at lower volumes?