First, LSD training and events are the worst thing you can do if you're interested in health, fitness and longevity. So, make sure you're clear on why you're training for a marathon. Now if you're slow you got to put in the work. It takes time. If you're older it might be to late. There's some really good advice down below about not moving to a longer race until you can hit a particular time in a shorter distance only the times are a little generous. This is the problem w/ this run, walk, run thing. It enables people who aren't ready for a distance to attempt it.
If you can't crack 35 minutes for 5k you have no business running a 10k. If you can't beat 70 minutes for 10k you shouldn't try a half. Once you can crack 2 hours in the half consistently you have permission to try a marathon.
