If I have totally missed the point of this please correct me. I am not sure if you are referring to a specific event requiring ability to hike through muddy ground or a normal everyday long hike. This is based on the latter, copied and modified from something I posted for someone else a while ago who was preparing for a holiday.
I don't know where you live and how easy it is to get to places to go hiking but this is definately the best preparation you can do for a hike. Get a small pack load and get walking. Keep you pace constant but routes varied so the intensity will vary based on terrain and gradiant. Over time build up the pack size and distance, if pace is a target, increase this gradually, by definition it will need to be something you can maintain.
It would obviously be more pleasant if this was done in picturesque surroundings where you can take occasional breaks to eat and drink, but if that isn't practical a varied route around streets and houses will do the same for you physically. I would say if going for the latter don't take binoculars, as bird watching is often frowned upon in built up areas.
Running and other training will help your fitness, but there is nothing beats hiking to prepare you for hiking. I have gone out with people who were very fit, a few of them marathon runners, and within 5 or 6 hours they were suffering from the packs on their shoulders and backs, tired legs, sore feet etc.
Practicing will also help you find out the best kit to use. There are some very expensive packs and boots out there, and I am fussy on the packs, but boots are always military surplus as the pricy versions give too much ankle support for me. Basically find what works for you and get it, unfortunately the price will have to be lowest priority, so if the boots you need are expensive, start saving.
First and foremost as always, enjoy it. Other cardio training can be virtually anything so if you really hate something ditch it and move to something else.
If this is for a specific event find out what the route and distance will be like and try to match it as closely as possible. If it's hilly with boggy patches try walking in areas that are likewise. If the most important thing is getting to the end fast and it's 10 miles with more techically demanding stuff than you have nearby try adding a few miles to make up for the easier routes.
Years ago when I used to run marathons for fun I would often take part in 18 milers, good way of avoiding hpoglycemia. One in particular was so gruelling I have finished at least one easy marathon faster than my worst time on that course despite being 8 miles less. So if in doubt on the difficulty add more distance and pace. It is better to have something left for a fast finish than not enough to finish it.
If the ground is likely to be wet and slippery you will definately want to work on core and co-ordination. There are wieros out there who seem to be able to balance happily regardless of footing on slimy or icy ground, I happen to be one of them. It isn't a miracle or big secret, hold balance at the core not the shoulders, knees stay soft and make small shifts. I learned a lot of this from ballet, and still maintain nothing is better for absolute control and co-ordination.