We were just contacted by Walgreens with interest in a pilot study using our mhealth self care platform. How do we cultivate this relationship?

Do you have anything to show him? Find out what his timeline is and what his goals are and see if you can deliver Also make sure this fits your own goals (Have you set any?) What are the delays in your development pipeline? Will this interest help resolve any?

I've seen too many enterprise-focused entrepreneurs mistake product/market-fit customer development with the pre or early-sales pilot stage.

To be clear, 1st impressions are lasting ones with enterprise clients at the pilot stage. Unless the company has such a pressing need that they are all willing to truly be a form of "partner" in exposing themselves to the frustrations of early product and everyone is aligned around that expectation, you are best to implement a pilot only when you have the confidence that your product can deliver.

A couple of other points for your consideration:
1) I think it's dangerous to cite a potential customer by name in a semi-public forum like this. Generally speaking, companies especially big companies are pretty sensitive about how their name is used in any context.

2) "Just contacted" is usually a long way (in my experience at least 2 months, if not 6 months) away from starting a pilot. Also, just because someone from a big company contacted you, doesn't mean they have the authority or influence to initiate a pilot. So if you haven't already done so, really qualify this person to understand where they really are in the decision-process and their role in the initiative, other internal actors and their relationship to those people and so on.

3) It's often best to form a customer advisory panel where you get one person from a company using your product in a cohort of similar people from different companies. This they can do "off the books" and provide customer feedback that can be synthesized with other similar customers. That way, you avoid building for what you think will result in a big sale for one customer when the broader market wants something else.

Happy to talk to you in a call if you'd like to explore any of this in more detail.

There are a lot of great answers above. I would like to add....
Be optimistically honest. Make sure your enthusiasm for the software and it's potential uses come through in your tone of voice and actions, but let them clearly know where you are in the process.
Good Luck!
Michael T. Irvin
NoHogWashMarketing.com
"Get To The Top Without The Slop"
michaelirvin.net