I did Unit 3 last summer and got full UMS; It is possible! You just have to put in a lot of revision. To be honest, I didn't really bother doing past papers in full as it's too much effort. A good thing to do might be to plan how you would answer past essay questions in your head during revision, it saves time and is helpful in linking between concepts across the course as you can flick back through your notes.
Have a read of the attachments I made from the core textbooks. They explain how to go about answering each of the questions. All the papers have a similar format from year to year.
A few more tips for the essay in particular:
Be synoptic. The concept looks hard at the outset but in reality it isn't and is not something to be afraid of. All it means is that you need to link in concepts from across the course into the essay, which is based under the heading of one topic. For example, say if you were discussing ecosystem productivity in an ecosystems essay, you could bring in climatic factors that affect productivity temporally or spatially, such as the quantity of insolation reaching the surface, which varies to the latitude, time of day or season at any given place. Alternatively in a world cities essay you could bring in knowledge of globalisation into a question about city growth and expansion.
Comparing concepts across different scales of analysis also gets you lots of marks; for example you might do a paragraph on how productivity might vary locally, then a paragraph about how it might vary globally.
Write with flair, and do not be afraid that you won't get the right answer; the examiners like it when candidates make insightful, interesting comments and back up their own opinions with evidence.
Good luck for the exam, and keep colouring!