I would try to avoid splitting a key workout. Definitely not a interval, tempo, or MP. I'd consider splitting a long, easy run only if time is a constraint. But supposedly to replace a 20 mile run, you need to do a 10 in am, 10 in pm, and 10 the following am. Heck, that's 2 extra showers!! So I wouldn't "plan" that but sometimes you're at 10-11 miles and life comes up ("come get your sick kid" phonecall) and well, it happens. I try to do doubles when I'm putting in about 10 hours a week. I sometimes purposely make the first run short, so I don't find myself saying "oh, don't really need it"<img alt="wink.gif" src="http://files.kickrunners.com/smilies/wink.gif">. I will also go to doubles if weight is an issue...I tend to drop pounds with doubles.