As far as fuel consumption is concerned, you will be flying an unfamiliar airplane with fuel gauges of unknown accuracy.
This is what I used to do for looooong trips....;
Make the first leg a 2 hr leg, note your altitude ,OAT and throttle & mixture settings,
Top off the fuel and calculate fuel burn/hr.
Make the 2nd leg a 3hr leg, note the same parameters, calculate fuel burn.
Make the 3rd leg a 3hr leg, same as above, you should now have a reliable fuel burn figure for that particular airplane.
Add .5 gallons/hr just to have a safety margin, eg 7.8 iso 7.3/hr.
I used to have a kitchen timer, engine start have it count down (in my case) from 4.0 hrs, when that timer reaches zero you better be taxiing on the ramp at your destination.
Using this method I have safely flown 4hr legs in a regular 1978 C172 and 5 hr legs in an equally ancient Pa-28.
When you plan your flights mark airports 0.5 hrs and 1hr before and after your destination, in case winds aloft go all over the place.
Take life vests and maybe even a raft, if weather forces you offshore you can easily get out of glide range.
Score yourself a reliable little GPS from ebay as back up in case your nav goes t***-up.
Make sure the plane has the required 100 hr and annual inspection before you leave.
Buy yourself one of these $2 Carbon monoxide detectors and check if the heater works correctly.
Some rental places will actually disconnect it to save on MX.
Make a written agreement with the owner/flightschool concerning unscheduled maintenance enroute, eg who pays for the starter/tires etc etc.