A group is definitely a plus if you don't have a build that can handle multiple enemies at once. You don't have to enter Cyrodiil in group, just find a group of guys at keeps and either follow them around or ask for an invite.

Running solo is very dangerous and I run into ganks all of the time when trying to get back to a battle. It's the unfortunate part of having to run all the way back to a battle if no one revives you. I'd suggest, after dying at a battle, to lie there for a bit and simply ask for a revive.

To get AP, you want to Heal, Kill, Revive, Defend Forts/ Resources or Take Forts/ Resources. Those all provide great AP. If you're struggling to assault enemy forts or resources, simply focus on defending your own. post up on a wall with a siege machine, that you don't mind losing, and try to take down enemies. After you fire, hop off and avoid being hit while it reloads. If someone takes it, which happens a lot, don't worry about it and keep going. Attack with ranged attacks, start healing and buffing people (even the bastard that stole your ballista) or setup another siege weapon and start pounding the enemy. Revives require Grand soul gems which can be expensive unless you're into the Vet ranks when you start only getting them in PvE. If you don't have a lot, be conservative with your revives and pick and choose who to revive. If you've got a Leeroy Jenkins kind of guy that keeps running under the Oil, maybe he needs to stay dead for a few minutes to learn that he shouldn't continually take hot baths.

It seems like all of the negatives are very true of Campaign but they're definitely dwarfed with the fun that you can have. You won't be having fun the whole time, unless you can get into one of the guilds that pretty much runs your campaign when they're on, but don't give up. If you really hate your campaign, just tough it out for a few days while you earn the AP. It shouldn't take very long if you simply are defending forts, rebuilding walls and healing allies.