VY
Maintaining the core training is a good idea and won't affect your marathon training although try and do it after your sessions.
If you're already weight training then there's no reason why you shouldn't integrate it. If your core routine is fairly comprehensive any non-leg / glute / hip weight work can be dropped (e,g shoulder press / lat work).
Lower body weight training is a good idea if you've already been doing it for a while, with these caveats:
1) Try and do it on the same day as a quality session, otherwise your body won't be optimising recovery.
2) Do the run first, weights after (can be later).
3) I wouldn't do a session after your long run.
4) Run short and very easy the day after a quality / weights day.
5) Unless it's purely an injury prevention session avoid low weight / high repetition sessions. Focus on high weight / low rep with plenty of rest. You're trying to build strength not replicate running fatigue in the gym.
Edited: 26/11/2012 at 10:29