Ride uphill and/or into the wind until you're crackered, then have a rest before flying home. Repeat maybe 3-5 times per week. When the outward leg reaches about 15 miles you can say the elastic's snapped and a longish day ride won't leave you wrecked for the week.
Edit, shove a chunk of malt loaf and a bag of jelly babies in your back pocket. If you hit the wall you'll need the fuel to get home.