Is Elizabeth surprised that Darcy runs into her in the park?

In reality, no. She may have been surprised initially, but after sharing that the park was one her favorite places, we can infer that she is nonchalantly extending an invitation.

More than once did Elizabeth, in her ramble within the park, unexpectedly meet Mr. Darcy. She felt all the perverseness of the mischance that should bring him where no one else was brought, and, to prevent its ever happening again, took care to inform him at first that it was a favourite haunt of hers. How it could occur a second time, therefore, was very odd! Yet it did, and even a third. It seemed like wilful ill-nature, or a voluntary penance, for on these occasions it was not merely a few formal inquiries and an awkward pause and then away, but he actually thought it necessary to turn back and walk with her.