Again, if he does that and runs away, he wastes their charge of sunlight. Hits and runs until he gradually wears them all down, with his regeneration outpacing any of the party's healing. Eventually they run out of healing, and they all die, without afflicting a single, lasting point of damage on Strahd.
That's not challenging or fun. It might be realistic, the way Strahd would actually do it. But it's hardly a way to end the campaign for a bunch of new players.
It would kinda stink, actually.
Strahd is trickier to run than any other Boss out there because he shouldn't be run like any other Boss out there.
1) He's bored. His universe is mostly shades and shadows. He doesn't have new things to entertain him, and that's part of his curse. There were several Ravenloft novels put out back in the day ("I, Strahd" is his personal story that has tons of source material to flesh him out), and for Strahd powerful beings new to his realm are playthings. He doesn't destroy his toys. Killing the PCs is the easy part. Breaking their morality is a task. Surviving Strahd can be a roleplay balance of keeping him interested in you (if he breaks you too quickly, he gets bored).
2) He's eternal. That can't be stressed enough. He can't die. Even if you destroy him he can't die. He'll just come back. The Dark Powers that created this prison from his darkness love the suffering. Fighting is boring to him. Corrupting another into hopelessness, the game is on. One of the worst parts of the module was the suggestion Strahd randomly shows up in the country and attacks ("haha, I'm a godlike being and you're 3rd level nobodies, haha, eat fireballs, haha"). Whoever wrote that bit hadn't read any of the novels. But if he ever gets to the point he has to "lower" himself to a physical resolution, and he's not the one who would ever start it, he'll finish it.
3) He's a wifebeater, sorta. It's a character flaw, his obsession with making Ireena / Tatyana love him through coercion and his dark powers. He wants to isolate them, make them see how awesome he is and how lame others around them are. Having a character flaw should play a part in defeating him, not simply be backstory fluff. Same with his diary. You've got a chance to give a treasure trove of insight and see what PCs can do with it. But, it makes what he's doing more exciting than combat. He's playing psychological games designed to break Ireena down, make Ireena feel like the reason everything bad is happening is because of her "choices," until the only choice is to go to him.
4) The unwinnable battle. This is the epitome of what distinguishes this Boss from any other. Every other Boss stands up, you dish out some HP damage, there's a phase 2 where the Boss gets some second wind or help, and the PCs dish out some more HP damage. His design, however, can frustrate players who expect every encounter to be balanced and winnable, because they have been conditioned this way through game design. Monsters are supposed to stand toe-to-toe and slug it out. Strahd plays to win and that means not playing "fair" for players. Beating him should be a major accomplishment, achieved through understanding how to overcome his invincibility.
That said, you indicated your players are "new." If they are used to the formula of:
monsters spotted --> roll initiative --> kill monsters, this can be an opportunity to chat about how Barovia isn't that place. There's going to be monsters who don't play by the rules, who might be way over your power level, and you'll probably die in a straight up fight. Or in the late Gygax's words about one of his dungeons, this is a "thinking (wo)man's dungeon." But don't make it too easy. After all, many have entered Strahd's realm. No one has ever killed him...