I agree with the high value treats - liver pate from a tube is my dog's highest value treat, though Primula cheese and dried sprats are a close second. For some dogs, a toy will be the highest value reward, or even praise from you (though I think dogs in the latter category are relatively rare!)
On a safety note, whenever you have your dog on a long line, make sure it's attached to a harness not a collar. The reason is that dogs can get up a lot of speed when on a long line, and when they suddenly reach the end of the lead, all that speed goes straight into their necks, which is uncomfortable at best and can cause trauma at worst. Before I realised this, my dog quite literally faceplanted the ground in this situation.
I'd also avoid flexi/extending leads - for two reasons. The first one is that the brakes on the leads can fail, and there have been cases of dogs being hit by cars in such circumstances, and they can cause injuries to humans and dogs alike if they catch on fingers or legs. The second is that the constant tension on the lead can encourage pulling.
Right now, taking your dog out into the park and expecting it to come back is setting the dog up to fail because there are lots of REALLY exciting things - people, dogs, picnics, squirrels, smells, more dogs... I'd temporarily give up on the idea of recall in the park for now (just walk on lead for now), and work on recall in an area with fewer distractions. Start inside one room in your house, then call the dog from one room to another, then try it in the garden, then a quiet park on a long line, then a quiet park off a long line, then a busy park on a long line, and finally a busy park off a long line. Don't move onto the next stage until your dog has mastered it in the less distracting circumstance.
Always give your dog a treat when it comes back. This applies now, and after you've mastered it - it's worth really, really reinforcing it because one day recall will save your dog's life - not an exaggeration. This also applies even if the dog takes a while to come back - as far as the dog is concerned, it's come back, so is due a treat. Never, ever tell the dog off for taking a while to come back to you as the dog will be less likely to come back next time.
And finally - you're not alone! If there are two things I think most people struggle with training more than anything, it's pulling on the lead... and recall 