You might decide to offer longer treks, for example, weekend breaks or week long riding holidays during which you escort groups of riders on day treks in the surrounding countryside.
You can either accommodate these guests yourself or offer to arrange suitable accommodation for them in nearby hotels, guest houses and B&Bs.
If you plan to provide board and accommodation yourself you'll need to give some thought to the type of accommodation this will be. It might consist of bunk house or hostel-type rooms designed with children and young people in mind or you might be planning a much more upmarket venture, with a very high standard of accommodation provided.
Think carefully about any meals you will offer and who will prepare them. If there are plenty of local pubs and restaurants nearby it might be more profitable to just offer breakfast, although if you are targeting, say, teenagers you will probably have to provide full board. This might include providing a packed lunch every day.
If you have sufficient space you might consider setting up a camp or caravan site - this could be used by both riders and non-riders.
You might be interested in offering overnight accommodation to riders who have nothing to do with your trekking business but who are looking for somewhere to stay with their horse. The Horses Welcome Scheme operates throughout the UK, and uses British Horse Society (BHS) personnel to vet the accommodation and to certify it's suitable for horses. The BHS website includes more information about the scheme.
Whatever type of accommodation you decide to offer, don't forget to obtain the appropriate planning consents and to make sure you are aware of all the legislation that you will need to comply with, such as fire and food safety regulations.