As mentioned above unless your lucky enough to already live in a place that has grazing and stable facilities near excellent / scenic outriding your going to need a huge chunk of cash and go house / land shopping first. depending on the size of your trekking centre depends on number of horses acres required also your own homely requirements to be taken in to consideration in terms of house size. Facilities wise most trekking centres have barn stalls to bring their horses into during the day to be groomed and saddled.
You personally would be adviseable to have qualifications in trek leading / BHS qualifications etc so you can whack your certificates on the wall and let people see your qualified and experienced so will be keen to book with you. considerations for a waiting area with toilet facilities and somewhere to gear up in the dry for your riders, tea and coffee facilities are a nice bonus in colder weather. plus your in Yorkshire lass we'll want a brew!
next step would be to plan routes look at picnic areas / local pubs suitable places to tie horses up, varied routes for different abilities of riders, optional extras like places for gallops and jumps / water crossings if available
Insurance for your business and animals is a must plus having good farriers / vets / dentists etc on the books consider first aid requirements at yard and travel / ride kits
there is a lot to consider the above is just some of the things to get you thinking about.
I know the YDTC was up for sale on rural scene a while back if I had a spare 800k I would of loved to bought that place stunning set up and nice outriding worth a nosy at if its still up on website will give you some lovely ideas
good luck and let us all know if you set it up you mat well get some hho visitors
If you are serious about starting a trekking centre I have available 8 stables right on the edge of the Trans Pennine Trail where you can ride straight onto the tops as far as you like with great little 'round trips' and a docile cob looking for some exercise which you are also welcome to let novice riders use! You might not need that small fortune after all!