Aside from anything else, playing games with your dog will improve the bond between you, so that your dog will have more focus on you outside. A lot of really excellent trainers use this approach.
I have high-drive gundogs and getting their prey drive under control is a job in itself, and takes a long time and a lot of input. But basic recall (away from sniffing, or in from the garden, or back to the car at the end of a walk) and lead walking shouldn't be too tricky.
Something else you can do is find ways to keep your dog's brain busy. For a spaniel, chuck some tennis balls into long grass and get him to hunt them up and bring them to you. Teach a basic retrieve and then make it more complex - he waits while you throw, you throw two things (like canvas training dummies) in different directions and send him to one at a time, you drop a ball and walk away with him and send him back gradually increasing the distance, you get him to sit and throw dummies over his head as you walk round him and then get him to bring them to you one at a time. This will give him a level of fulfillment that just running about sniffing doesn't provide, and should make him easier to handle.
All the gundog trainers I know, including the ones I train with, use balanced methods (rewards and punishments, but 90%+ is rewards, and I count a punishment as a check on the lead, for example). If you are not comfortable with that, that is your shout entirely, but the dogs I know who are trained this way are confident, happy and fulfilled and have excellent relationships with their handlers.