The books seem to tell me that to gain endurance you don't have to run fast, and often recommend a pace 90s to 2min/mile slower than race-pace for long runs. That's around 6:30 to 7min/km pace if race-pace is a 4h marathon. My avg. pace was 6:48/km today at avg. HR 154. A recovery jog on Monday or Tuesday, and then a faster 5k on Wednesday or Thursday - that should be about the final prep for the half-marathon next Saturday.
This scenic loop is close to what they use for part of the Helsinki City Marathon (2009 map here).