I apologize in advance if this insults your intelligence as I don't know your skill level.
If you want some "repair" advice, try this:
Dust the thing off. A cleaning with dishsoap may be a good idea. Don't use a high pressure hose, just a bucket and sponge. Rinse with a stream away from the BB and hubs (cassette side). Dawn dish detergent used full strength on the chain is an excellent degreaser for standard lubes. A wax lube is much more difficult to remove. Towel dry or air dry really well.
Personally, I would pull the tires and tubes and trash them. The tubes "may" be ok but you don't want to chance that on a 70 mile trip. Are the wheels 700c? If so, some of the online bike stores have REALLY good deals on tires (and tubes). Buy both and save on shipping. Average bike store tire costs are $20-30. Tubes run $4-6. Cheaper on-line. You can buy a 3 pack of tubes to keep a spare w/you. I would suggest buying a CO2 inflator as they are small and relatively cheap. IMHO, better than a small frame pump. When installing the tubes, put a little baby powder on it to help prevent it from sticking to the tire. Makes changing it easier if necessary. Make sure the rim strip is in good shape.
Using a small piece of 000 steel wool or scotch brite pad, clean the braking surface of the rim (easier done without tires installed). Using a piece of fine sandpaper (after removing the brake pads from the calipers), clean all the glazing off the pad and refresh the surface. Unless the pads are completely shot, this can extend their life some and save you some cash. You can adjust the cables and such to get the feel you want.
Not much you can do for a rusted chain but replace. If the chain is worn, you will be able to measure more than 12 inches pin to pin. If the measurement exceeds like 1/8 in, replace the chain. If this is the only time you really plan to ride, you may be ok but may suffer shifting issues and excessive wear on the cassette. You should replace the cassette when you replace the chain but you want the cheaper route. Once you have all this figured out, use a good lube (Triflow is a good wet lube). Spray on then wipe off the excess. The point is really to get the lube into the chain, not on it.
Check with your local bike dealer on a "used" seat. Some people upgrade when buying new bikes and they may have a slightly used one for cheap. Nothing out there is gonna help the pain after 70 miles though. We did 103 on Saturday (third century this year already!) and I'm still feeling it some.
AFTER you do the repairs, unless you know what you're doing, take the bike to a shop and have it checked out. They can adjust cables and shifting and such to make sure you're not going to get stuck in one gear!
Tires - 2 for $20
Tubes - 3 for $10
Chain Lube - $7
Seat - $35
Inflator - $20
Tune-up - $25
You may be able to bundle all the parts from the on-line stores and save even more than buying from a shop. Nutrition on the bike is important. Hydrate and EAT. I don't know what kind of average you're looking to put down but you have to eat. Even at 15 mph, you're gonna be on that seat for almost 5 hours. GU gel works well for me as do Clif Bars during the stops. I eat a banana halfway through longer rides for the potassium.
And MOST importantly,
ENJOY the ride!