This is why planning and design is important. Make a quick Game Design Document, and list all activities you need to do for your prototype. Only you know how many activities you can do each day. Here is my list example:

1. Player Movement
2. Player Aim
3. player attack
4. Basic map
5. enemy behavior
6. damage system
7. repair system
8. loot
9. inventory
10. hunger system

So I have 10 main activities and 14 days minus planning. Now I now what I need to do. If i have an awesome new idea, I write it down on paper, and put it as an additional task, which I'm not gonna touch unless all the other tasks are finished. If finding a tileset is important to you, define a specific task to it. It should not take you more than what is planned, otherwise your schedule will suffer. We usually get lost in time because we don't have a defined path, and later we realize we needed to do more stuff to have something playable.

Also, don't try to be perfect. A game jam is not for creating a full game. It can be, but is not important. Put your mind to work on finishing a prototype. Art can be ugly, game mechanics may not be perfect. What is important is to create a good idea of what you are trying to achieve. If the idea is good, you can keep working on it later. Best of luck!