Handling Long Proccess

Ok, I have a php page that takes a long time to run, its a lot of external api interaction and database transactions. Normally this will run as a chron job, what is the best way to make sure this runs its whole length without timing out? I've seen pages run the "Maximum Execution Time" before and I dont want that to happen with this.

Also, I would like to be able to manually start this proccess from another page, what is the best way to do this? I mean, I could just execute the code from this page, but then that would make the page wait untill proccessing is over.

Any help would be awesome.