Helen, well done for what you've done so far!
The biggest trick with a marathon is to run steadily. 5 hours relates to about 11:30 per mile, and on race day that would equate to half way in about 2:25 to 2:30. If you're doing 5 hours by running the first half in 2:00 to 2:10 (about 9 or 10 minute pace) and then hitting the wall, then you're more like us!
I don't know anyone who can "only" run at 11 to 11:30 pace. Even the age 60+ men and women I run with can still get faster than that when they're thinking about it, so it might be worth trying some longish runs at about 10 minute pace and see how you do. Build up the mileage very slowly at this pace, as well as shorter runs slightly faster during the week.
Much of any one's success in the marathon is running through (or just avoiding) the wall. By conditioning your body to run far, regularly, you're extending where that point is. Remember when you couldn't run more than 5 miles? All you did then was run a bit further each time, and now you're better.
Keep up the mileage, good luck!
(and stay here with us and we'll get you through that 4:30 barrier too!)