To answer the question I constructed a graph of Lizzie's speed distribution, how much of the time she runs at different speeds.
From this graph we can tell a couple interesting things. First it appears that her sustainable top speed is about 2.5 mph. In addition while she can go that fast she prefers to run at around 1.85 mph. Interestingly enough her time running at each speed looks like it might be a standard distribution around that center point slightly skewed because of the ramp-up necessary while she accelerates from a stop. A slightly more detailed graph is available here.
On a related note, we love questions! Please feel free to keep asking them in the comments or by email. We'll do our best to answer them and feature the best ones in the FAQ and on this blog.