I am in a similar situation. Just completed my first half marathon in Nashville on the 26th. 2 days later I was signed up for the full Twin Cities Marathon in October. And like you, Im doing an 18 week training program. So what am I doing for the next few weeks?  Well, my long run this week is going to be 5 miles and then next weekend Im doing a 10k (6.2 Miles). That race is a little bit different as in half of the course is off road on some grassy walking trails. So a bit of cross country running with a natural 5k. Should be fun, but yet keep my miles up there for week 1 of the program.

Which marathon are you doing in October? 

Good luck on everything!!