the decay wil only take it as far as the 808 goes. another thing u can do is turn the pitch down, but then of course, it will be lower. if u have battery or something that lets u manually timestretch the wave, u can timestretch it so its longer without messing with its pitch.
I don't know if you absolutely want to keep the short 808 sample you've got and make it longer but there's some VST's out there that can sort of emulate the TR-808, you can tweak the attack, decay, release, pitch.... etc.. to your liking. For example you can try Drumatic 3, it's a free VST you can get here http://www.e-phonic.com/plugins/drumatic3.php