In the past I combined maps by highlighting the projects I want to combine, and exporting them as I would any project. Make sure they have the same properties and water level offset. I had an inadequate computer I used for a couple years that would not generate a single large map, but would allow me to do it this way. I also had to be sure my map boundaries (shorelines) overlapped a little. In the end it was awkward, and so I carried multiple cards with multiple maps.
I recently bought a laptop with 16 GB Ram, dual drives, 2 GB dedicated graphics, and a 64 bit system, and now I don't have to do this anymore! It runs brilliantly creating a huge shaded relief AT5 map in 47 minutes. I have 1,500 trails.