1999 Jeep Cherokee Jeep won't start, fule pump or battery?

I have had my 99 Cherokee Sport for about 18 months, and it has always taken 2 cranks to get it started (unless car was just shut off, then it cranks right over) I noticed yesterday when starting it it sounded like the battery was going dead, and sure enough this morning it wouldn't start at all. Tried to jump it several times, finally got it started but unless I had my foot on the accelerator it would die again. My dad actually had to manually engaged the accelerator under the hood to get it to start also. So it won't idle after being started, and tryign to jump it didn't help either. Could this be just a severely dead battery or could it be related to the fuel pump somehow, or maybe related to me having to always crank it 2 times to get it started? We had it checked over by a very honest mechanic & he couldn't find a problem to explain having to crank it twice. I don't know how old the battery is either, was in jeep when purchased.
Any ideas would be great.