I would respectfully disagree @Shopgirl1...
Interval/speed work is good for anyone training for a marathon, regardless of whether you have a time in mind or not. It's different on those (annoying) threads where people come along and say they've signed up for a marathon, haven't really trained properly, and want to know how they'll get round, but the OP specifically talks about 'training for' her marathon.
Speed and interval sessions don't have to be some highly technical elite runner type session, but they work your legs (and CV system) in a different way, they break up the monotony of training, and they hugely help your running fitness. I would really strongly consider incorporating this into your training, given you have so long and are taking a very sensible/planned approach to it.
It can be as simple as finding a hill in the middle of your run and then running up (and down) it a number of times, ideally pushing yourself a bit going up the hill (and using the downs as a recovery). Or using lampposts and aiming to run faster between posts 1 & 2 then recovering between 2 & 4, then going faster again. Or running between 2 lampposts faster, then later in the run going fast between 3 lampposts, and later on between 4 lampposts and so on.
Or a pyramid-style session where you run fast(er than normal) for 1 minute, then recover for 1 minute, then fast for 2 mins, recover, fast for 3, recover (repeat up to however many minutes you feel like), then come back down the time periods again back to 1.
All these will hugely help you in your training. I wouldn't start doing them until you're safely out the other side of C25K/are happy running 30 mins, but after that add them in and you'll be surprised what a difference they make.