If your dog tows you along it its wake, it is time to do something about it. If you always let it go way ahead of you on an extending lead, Fido probably thinks you've promoted him to Pack Leader, so he will always want to do that. You need him by your side and under control. Long-leads have their place, but that is not while walking along the pavement/sidewalk when you should have better control. More likely, however, is he is just so eager to get to the park he can't wait for you. You need to make him realise pulling is not the best plan.
Methods to stop this include stopping suddenly, then waiting for the lead to go slack before starting again, turning around to reverse the expected progress, treat training combined with the above, click treat training, or using one of the really kind and effective non-pull harnesses on the market - but not the 'wrong ones' which can injure your dog.
I've posted an article on my website on this subject which suggests 5 detailed solutions - including the harness details! I hope this helps you get to enjoy your doggie-walks a whole lot more! For the full article go to: http://www.iankingsley.com/articles/004-stop-dog-pulling.php.