Agree with metalbryan, 50-60 miles per day is a good pace for long distance travel. Depending on your fitness level you might start at 35 miles per day and build up. I normally took 1 day off per week unless there was bad weather and might take another day to let it pass.
For pacing, I always averaged about 10-12mph moving speed. When you factor in breaks, lunch, filtering water, hunting for a campsite, I would figure on a 7mph pace throughout the day. So if I had to go 50 miles, I expect it would take 7 hours from leaving camp in the morning until arriving I'm camp in the afternoon, that would include all stops. So at that pace if I had to do a long day because of lack of water or places to camp, i would expect it would take 10 hours to cover 70 miles.
I would also look to make sure the bike is serviced and you know how to replace all the parts. Mongooses come with cheaper components, so expect to replace a pedal, bottom bracke, or even spokes in the first 1500 miles or so. Luckily bike parts are cheap and if you hit up co-ops you can use their tools and get parts for really cheap.
