Taking on another persons job while still doing my own?

LegacyUser

I work in a pretty small company. I was very comfortable with my job, but do admit that I always ended up with a bit of free time each day(I would offer to help other people in my department at those times).

I work 36 hours per week and my own work on a typical week would only take about 30 hours I'd say. Another person in a different department was recently let go. My manager informed me I would be taking over their duties as well as keeping up with my own. The other person worked 30 hours a week. I am still only working 36.

The other job is completely different to my own work and I have no experience in it. I received 4 hours of training from the person who was leaving. I have no clue what I am doing and am very concerned about making mistakes and getting the company and myself in to trouble. There is no one else here that knows how to do this work. I assumed my boss did, but apparently not.

I have expressed how I do not think I have enough training/time to do this extra work, but all I am getting is a few hours of help from interns (which is next to useless as I don't know what I am doing myself, so it's not as if I can teach them) and no proper solution.
My original work is starting to suffer too due to lack of time.

My contract states that I will work in any department. I guess I stupidly thought of that as helping out with easier tasks in busy periods.

Any ideas on what I can do?

Bandana boy

Grab all the intern hours you can get , train them to do your existing job so you can concentrate on your new role.