Don't always count on your PCs running from a fight. Many a time has it been for me where I said "There's no way they can... How on earth did they DO that?"
To answer your question specifically if you want to do the chase maybe the Chemist can take a swing at them when he catches up, regular attack vs AC type deal. That way the player's won't feel like its an auto kill, especially if you do all such rolls in the open.
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Not sure why specifically you think they can't win the fight. Certainly there can be very strong builds or tactical synergies that change whether CR is a good measure of threat, but a a CR 7 encounter should not be overwhelming for a party of 5th level characters unless the party is depleted of resources, small, or ineffective for their level.
That all said, a mechanism that is appropriate for the party to recognize that they are in a problem encounter is to use Snbse Motive to communicate the enemy's confidence. Additionally, dropping a foreshadowing element prior to the encounter can generate that concern. Finding the corpses of a prior encounter, maybe including the body of an NPC acquaintance who was know to be more capable and cautious about travellg or adventuring with other effective NPCs might do the job.
The key is to give them appropriate clues and let them draw the conclusions.
I've only run a chase scene once, and that was so a group of pirates could evade the PCs. If you really want them to get away drop a scroll of Haste in their path and that'll help them speed away, probably faster than the Chemist can keep up.
Hope it goes well.
please report back after the session and let us know what happens. I'm curious to hear how you convince the PCs to run, how they do it, how the fun was had, etc.