I'm in a similar position, Ben. Doing my first tri's in the summer, and then Chester marathon in October. I'm focussing on tri training until July, and then I will 12 weeks of devoted marathon training plan. I would have liked a 16 week period before Chester, but I reckon the tri training will have got me fit enough by then.
My tri-training is based (loosely) on doing something 6 days out of 7, and a few double session days. Swimming at lunchtimes as often as I can (2 or 3 times a week), running 4 times a week (one longish hilly run, one parkrun for speed, two tempo runs). Bike is limited to turbo sessions at the mo, 3 or 4 a week.
Once the tri's are over, I'll follow a Smartcoach schedule.