I recently visited a country estate in the UK. The gardens had a number of large beds and the paths through these beds where all lined with low (2 foot or so) fences to create the barrier. The really clever thing was that these "fences" where made up of apple and pear trees, trained to grow along a series of supporting posts. The trees where covered in large, juicy fruit and from what I could tell each tree was trained for 10's of feet in each direction to form the fence.
I have never seen this done before and wondered if anyone here had any ideas for varieties of apple and pear trees that would be suitable to grow in this way to eventually form the main paths of my allotment patch. I am also looking for any suggestion on the best techniques for getting this kind of long low horizontal training?
These sound like trees trained as espaliers, though espaliers usually have several horizontal "layers" to get more fruit with a restricted height so that it can all be picked from ground level.
For good results you need to choose suitable varieties, grafted onto suitable root stocks. Note this is not an "instant gratification" technique and it will take a few years to train the trees as you want them. You won't be able to buy a "ready made" espalier and just dig a hole and plant it!
There is some general information here but you would be best to contact a specialist fruit tree supplier to recommend what will grow best in your location.