He is a puppy, and he will grow out of the distraction/excitation somewhat as he ages, BUT what I would do right away (and am doing with my own pup, as we plan to eventually bike, jog, hike & horseback ride with him) is to have 2 phases to every walk (or do separate walks, depending on time/weather).
Phase 1: regular leash- brisk walking pace where pup is to stay "right here" which is a relaxed heel position, or "heel". Some sniffing/distraction is allowed with "right here", but NOT with heel, but he must stay right with me on both and is not allowed to dart off, lag behind or the like. He is praised for keeping the leash loose, and ack acked (verbal correction) for pulling and darting.
Phase 2: off leash or long line/retractable- he is told to "go ahead". This walk is all about what HE wants- sniff, romp, run, roll, whatever he chooses. The only requirement is that if he is off leash he MUST stay within eyeshot at all times, and must recall instantly when asked.
Most walks at home are a mix of the 2- phase 1 if we are outside of the fenced farm area (our road in front of the house for example) and phase 2 in the fenced yard or in the fields where I am able to closely monitor. Most walks at work are phase 1 to and from the park, and phase 2 while at the park for a majority of the walk.
As he gets older (he is not even 5 months yet), I will expand on the phase 1 training to include off leash compliance and start incorporating walking next to the bikes, horses, etc... Eventually, as he is old enough to join us on our real activities any that are on the farm will be leash free with "right here" to maintain close contact/recall- and leashed phase 1 style when in public (on the rail trails or state parks).
My prior dogs were leash free 98% of the time and would maintain perfect heel or "right here" with no issues. They knew that at least part of every walk allowed some "go ahead" time- and they were willing to wait for it
Hope this helps..