Divide implementation into short periods called iterations, and keep track of how many stories you get done in each iteration. The pace at which you currently develop stories is an excellent predictor of your future pace.
Track the tasks you do in each iteration, and note how long each one takes compared to how long you expected. This gives you an even better predictor of your day-to-day ability to implement tasks. Since you can estimate tasks better and better, and since stories are made up of tasks, you will learn to estimate stories better as you go.