Working out is always a good option, but as long as you want to be doing it. Do it as much for yourself as for her. Although don't feel like you need to go nuts and aim for a six pack if that isn't your ideal aesthetic! It doesn't need to be all or nothing.
Personally, abs have no input on whether or not I find a guy attractive. My current boyfriend has no fat or muscle to him at all (if anything, he could benefit from a pie - or a dozen); and I've also dated a guy more than double my body weight and, no, none of it was muscle. Plus guys of varying body shapes in-between. I've been attracted to all of them physically regardless.*