I do not count on RA to have a fleet because, as Hack78 said, you WILL get owned if your opponent realizes you do not have a fleet. My approach is slower but in my opinion has a higher chance of succes.
I use what you would call guerrilla tactics to destroy assets a normal player would overlook, such as construction ships. structures not yet built and the like. I guarantee he will be absolutely pissed, will subestimate your measly force of five frigates and will try to find you, but that is where your capital ship will come in...
bwahaha!
I have found that if you launch repetitive short, fast attacks against your opponent's planets, he will play defensively for a while and slow his advancement long enough for you to reach RA level...
and terminate him for his mistakes!