Parking is a bit adhoc. Sometimes there is space in the sports ground parking lot, but often people just park on the side of the main entrance
The pathways here are mostly earth the trees are mature and provide a good bit of shade. Along the river the paths are paved, it is possible to walk all the way into Boston along the river.
There is a river, sometimes there is a fence if there are docks or boats but othertimes there is no fence between the path and the river.
There are poo bins scattered along the path in Witham park mostly along the paved sections but not exclusively
The view of the Boston stump when you walk along the river towards Boston is lovely. You pass the Boston Lock Cafe at about the halfway point if you start to feel a bit peckish or you can use that as a turning back point and head back through Witham park.
This is one of my favourite walks in Boston, Witham park is a mature park with great opportunities for rambling through the forest and along the river. There isn't too much wildlife to observe, just the occasional squirrel or pigeon. The dogs are safe to come off lead in several areas of the park, while walking along the river use your best judgement though as that pathway can be quite busy with other people cycling or jogging etc.
Tip: there is a park run that happens here every Saturday morning which you can join with your dog, or aim to avoid if you like a quieter walk.