It's a bit difficult to say because I don't know the level of the students, I haven't actually seen the material you are going to use, and I haven't seen what the "task" actually consists of.
But, I used to have problems with organzing my time. Some activities would go over the time I thought they would need, and we would get off track and lose time, etc.
Now, I break down my lesson plans into estimated time increments, like this:
10 mins - warm up; "Describe hometown to partner"
5 mins - Explain directions
5 mins - Discussion of task
10 mins - Ss brainstorm in pairs
5-10 mins - regather and discuss as group
Right there you have about half an hour!
I always overestimate by a few minutes because (for me) it's better to have extra time then to run OUT of time. After you write it out, take a look at it. Did you schedule everything? Can you add some minutes to some activities? Can you take away some minutes in another? Keep an eye on the clock, and really time the class.
It may not go "by the schedule" the first couple of days. You may have to take a couple of classes to find out the pace of your students.
Things go so much more smoothly now. This is a good way to keep the students on task and active.
I hope to have helped in some way. I can't wait to hear more suggestions.