Adding another session

I'm at the start of a fairly focussed training programme for a Spring marathon (my first), and whilst I am enjoying a 3 session a week programme at the moment, I feel I could do with adding a 4th in the next few weeks.

Currently, I am running a short  easy run (5-8 miles) on a Tuesday; an tempo / speed session on a Thursday (5 miles), and a long run (11-13) miles on Saturday or Sunday. Each session is getting slightly longer or more intensive each week (e.g. the long mile is increasing a mile each week). I'm also trying to fit in a gym session, focussing on leg and core strength.

 So, in terms of a 4th running session, when and what should I do? I recognise it should be fairly easy, but when should I fit it in? Does Monday sound about right (ie. 2 days after the long run)?

I would suggest a Wednesday and make it another long run but shorter than Saturday/Sunday's run. It'll support it so that the long run doesn't become unreasonably difficult in itself.

So if you're doing 13M, 8M or 9M midweek is good, and when you're doing the marathon-specific long runs (20+) 11-13 will be needed or you'll find the long runs are killing you. It seems a bit of a contradiction at first but you'll find the longer you can run midweek the easier the longer runs become when it comes to the crunch time of marathon training. The sub-3 lot sometimes do 16 midweek when they're doing 23-26M long runs but you can get away with 11-13M IMO. 

Cheers Duck: how would I begin to integrate it in? Do it the day after the short-easy run, and allow a rest day after? Start it at say 8 miles, when the weekend run gets to, say, 14 miles, and then increase length by a mile a week up to 11-13?

Starting like this?

M - Rest / X-Training

Tu - Short easy run (8 mile)

W - New midweek long run (8 mile)

Th - Rest /XT

F - Tempo run (8/9 mile)

Sa- Rest / XT

Su - Long Run (14 miles)

That looks a decent and well-balanced week (although Tuesday's run could be 5-6M instead of 8). I'd say you could get away with 9 or even 10M for the midweek run at this point though, and increase it in tandem with the LR but with a smaller increase each week or even every 2 weeks. So you could go 14/10, then 15/10, then 15/11, then 16/12 or something similar. Ideally if the MLR is about 70% of your longest run then you should be ok.

If you wanted to go a bit further I'd also suggest adding another easy run on on Thursday with some strides. Wouldn't have to be long, 40-45 mins tops. 

FWIW I'm doing 13-15M long runs now with 10M on Wednesday, although I'm not marathon training so it's not necessary for me to go much further than that midweek. If you can keep that kind of ratio between the LSR and the MLR then you won't go far wrong.