There's a book, Run Less, Run Faster, that has plans incorporating cross training. I bought it a few years ago but decided against it as I'd rather just run. It's curre sitting on my book shelf gathering dust, so if you want it feel free to PM me. Happy for it to go free to a new home!
I'm currently following a plan from Pfitzinger and Latter's book, Serious Road Racing. Although I was running faster previously, I lost my speed last year as I had 5 months off through injury and did nothing during this time other than yoga and Pilates.
I've followed the 12 week 5 k plan - the easiest being up to 45 miles a week - and have just completed my first tune up race. I ran my best time since the end of 2014.
I'd recommend trying their plan if you can commit to 5 runs a week as the combination of consistent mileage and targeted speed sessions should bring your times down.
I've run 7 marathons and for the first 6 struggled and lost up to 10 minutes usually on the last 6 miles.
For my 7th I followed the Pfitzinger & Douglas 12 week/55 mile plan, aimed for my second sub-4 marathon and ended up with 3:45 and some seconds, with a 9 minute negative split. I was delighted and finished feeling strong. Could I have gone faster? Who knows???