If the axle hasn't been out in a few years it can rust to the bearing spacer in the hub. If rusted very badly it can take a lot to get out.
Soak well with the penatrating oil you like best. Put the bike close to a solid object. Like the leg of your work bench. Put blocking between the fork and solid object. Thread the nut on so the end of the axle is flush with the nut. Use a block of wood to protect the axle/nut. Now hit with a big hammer. If it moves a bit then move the blocking and drive it back in. Work it in both directions untill it comes out.
Get some anti-sieze, clean the axle well, clean the inside of the bearing spacer. Use plenty of anti-sieze on the axle when installing.
Leo