What kind of analytics program are you using? Knowing how much time someone spends on your site, is a good start, but try and dig deeper into your data to see what other information you can gather.
Look at similarities between your past conversions? What did they have in common? It could even be a certain browser or screen resolution. Start thinking that deep, you can never have too much data. Follow what Primal says. Look at your landing pages, but break them into chunks like headline, call to action, button color, etc...
Take one chunk and try to optimize it, then like Primal said, document everything. Look at it like your high school science experiments. Don't make too many changes at once, and be sure to implement your changes for long enough to gather enough data to make an informed decision.
Once you feel like you reached a good threshold for your landing pages, then look at your PPC account. It doesn't make sense to change keyword bids up or down, and make changes to your ad text until you're sure that your destination is well prepared to convert those keywords and ads.