If you want to improve your power over that kind of distance (5 miles) then you need to be doing lots of threshold efforts and V02 max intervals. Suitable training platforms are Wahoo SYSTM or TrainerRoad. Those have the best online plans for this kind of thing without trying to overthink it. Just choose one of their TT focused plans and follow it. It's very easy to over-train at threshold or above, so pay attention to recovery. Decent plans take recovery seriously.
Edit: You might want to look at your position on the bike too as aero will play a significant part in your average speed for this kind of segment.