I'm nearly up to Level 24 and am enjoying it more as it get harder because the rewards are better. Just keep catching those silly ones for stardust and visit the Pokestops, it's surprising how soon your XP level goes up. Have a pop at gyms and stick your creatures in (took the Tower of London one today, memories of Arsenal boxing day '77), get coins, buy incubators, hatch eggs (you never know what you'll get) and see what's nesting or spawing in your area on the web. A couple of visits to Blackheath over the weekend has got me a Charizard and an unexpected Lapras as an example. Don't rush to evolve or power up low or mid-level Pokemon, maybe run them from first to third stage in one go if you have enough candy from a good starter by checking it's IV online.
Sometimes I wish I could start again and do it all differently, saying that though I've tended to dump the earlier catches and evolutions for stronger ones I've caught as my XP level has risen.
All done honestly too, other than following the tracker hoards around Blackheath, who would'nt?