Ok!
Here is a selecton of thoughts, suggestions and questions.
1) Are you being paid as HoD for acting in the role on a temporary basis?
2) If you are not being paid to act as HoD, then your SMT CANNOT expect you to do the level of work you are doing for more than a couple of weeks. We are all prepared to do extra work to cover missing teachers for a little while, but not in the longer term.
3) If you are not being paid to be acting HoD go to your SMT and state the position. Explain exactly what you are being required to do and tell them what you want. You have two options:
a) "I will act as HoD for this term until a replacement comes, but I want the money and reduced timetable that goes with that job."

"I do not want to HoD for this term. It is not my job, so someone will have to come into the department from a senior positon to sort things out. There are too many inexperienced teachers for me to cope with."
Of course they may not want you to act as HoD, and in that case you can remove yourself from the role, but in a more sensible way.
If you do decide to offer yourself as acting HoD, then you may have to accept that the 7.30am til 6pm day will be your norm in the short term. If you are going to have to plan for the whole department this is what it will probably take.
4) Get worksheets and materials from the schoolhistory website. Lift them wholesale and write your lessons around them. Do not worry if your lessons seem to not be covering enough and you are going to fall behind the S of W used in the past. You ARE in a less than postive working situation, and to produce lessons for all years at this stage is tough enough.
5) Use exercises straight out of the textbooks. I know we all rubbish therse most of the time, but they can be used and will cut down on planning time.
6) HOLD DEPARTMENTAL PLANNING MEETINGS. Get the whole of the department (probably not the Head of Sixth Form due to time) round a table once every week and brainstorm ideas. Lesson plan together. This is one of the best methods of getting everyone on board and making sure they all understand what is needed. You will doubtless still have to do most of the donkey work, but it will make it easier.
7) Seek out friendly HoDs and talk to them. Ask them what you think are the stupid questions as well as the hard ones. They will not mind! They will understand the school situation and many will be only to glad to help.
8) Consider contacting the Head of History at some local schools. They may be able to come and see you after school one day, or vice versa. This is a more local contact and could be extremely useful.
I could say it will all go away, but by the look of things it will not. The most likely scenario is that you will get a new HoD in January, so you will have to do the job until then. If you think you may be ready for the challenge, then now is your chance to show it.
In addition, people such as myself have lesson plans that I am sure we could e-mail to you. If you want to e-mail me using the www link at the bottom of this post, I can doubtless send you some of my specific lesson plans that might be of use to you.
Good luck!
"The whole problem with the world is that fools and fanatics are always so certain of themselves, and wiser people so full of doubts." - Bertrand Russell