Steve, what type of surface were you on for the 4 mile test ride? If it's road then you might find having higher pressure in the tyres will help - a pump with pressure gauge will help. Or try more road focused tyres.
If your planned ride is this one http://www.bike-events.com/Ride.aspx?id=366
you'll notice that in the rules it says that it is not a race. But it also says that it is 60 miles.
If I were you I'd be getting a few miles in on the bike. Make sure that it fits comfortably and move the saddle up/down, nose up/down, and back/forward. Similar with the handle bars, as bfergie said. There's nothing wrong with messing about doing this, just take a multi-tool on test rides and be prepared to make adjustments on the roadside.
If you haven't already, consider getting clipless pedals as you'll be able to pull up as well as push down - makes for a smoother pedal motion and shares the work across your leg muscles.