Doing one run a week that’s significantly longer than most of your other runs is a great way to simultaneously boost your mileage and your fitness. Your main motivation for regularly doing long runs should be the latter—building your endurance so that all of your runs become more manageable. But I’m not going to deny that starting the week with a long run is a killer kick-off to meeting your weekly mileage goal.
Near-weekly long runs are, of course, one of the backbones of marathon-training plans. Almost all runners looking to race well from 5K on up know the value of the long run. The internal changes in your muscles caused by long runs—basically, a vastly improved “plumbing” system for getting oxygen to and removing waste products from working muscles—carry over to all your runs. There’s also a great mental benefit from these runs devoted to making yourself more resistant to fatigue, in that your normal runs won’t seem nearly as daunting. When you’re used to a 2-hour run most weeks, a 45-minute run on even the most stressful day of the week is easy to imagine.