First of all, 1.5 is outdated, 2.1 and upwards should be the focus.
Second of all, programming and running a forum software are two different things. You don't need any programming knowledge in order to run the forum software. Difficultywise those 2 things should be seperated, too.
Running a forum depends on what you want to manage yourself. You need to buy a domain, you need to purchase a license for XF, and then you need to rent a server to host your forum.
Now the difficulty in buying a domain and license of XF is pretty straight forward, which means easy.
Server aspect depends on. Most go with shared hosting solution where you don't need to manage anything at all. You have mostly a panel where you can configure stuff of course and where you can upload your forum software or download backups. This is also very straight forward although a bit learning is needed.
Then you can go with a managed VPS, where it is basically the same thing as shared hosting, where you don't need to do anything yourself, but you get more resources (CPU, RAM, Space) for you but it is more expensive.
Unmanaged VPS means you need to take care of everything, so installing PHP etc.. I would say difficulty-wise it is high difficulty for a novice.
Unless you go with the popular choice
Centmin Mod, which takes care of everything. It is a mod you install on the server and then the rest is straight forward. I would say 1-2 days and you get the gist of it. There is a learning curve (you need to get comfortable with SSH access).
Programming is not needed to run the software as I said. But if you need custom solutions what the forum software cannot do natively (or you don't find any addons which suite you or you don't trust them), then you need to hack the software by creating your own addons or other stuff (scripts). The difficulty is enorm. I would say a dedicated man can learn the basics in 2 weeks and to be able to create addons you need more time and dedication. A template modification? Sure, in 2 weeks. A big fat addon? Probably 2-3 months to understand the system.