Southend Bikathon again

Here we go again! I have signed up for the Southend bikeathon (26 miles) this was my big ride last year and so it's encouraging to see how far I have progressed since then (even though I didn't cycle for about 6 months after the bikathon!). The big question now is, do I do the ride on road bike to see how quickly I can cover the route, or do I ride one lap (13 miles) on the mountain bike with Seri in the trailer?