Dan:
1. I prefer going up a few miles each week with occasional
easy week ie 15, 17, 19, race/15, 21
2. most feel it should depend on your speed and your
training miles but a run of 21-23 miles is long enough for
most people but it should not be too fast.
3. There isn't a rule and it's down to personal preference,
opportunity and fitness but 3 runs of 21-23 would be what I
would ideally aim for.
4. Three to four weeks for the last longest run. some
people like to do a half-marathon race three weeks before
so the longest run goes 4 weeks before and the last 3 weeks
should be tapering so I like to run around 15-18 2 wee4ks
out.