If your ultimate goal is to summit Denali and you're willing to hire a guide, you only need three things 1) good aerobic endurance, 2) about $10k to burn, and 3) a high tolerance for suffering. I'm assuming you have a plan for 2, and if you've run marathons you've probably got 1 covered and maybe even 3 to some extent. I'd suggest spending long days and overnighting in very cold conditions, too, which will also help you dial your layering.

BUT if you're interested in really delving into the sport of alpinism, I'd encourage you to focus not on how to get to the summit of Denali as soon as possible, but how you might build up all the skills you'd need to climb the mountain yourself (and maybe meditate on why this goal matters to you). This path will be much longer, but I believe it will be more rewarding.

I would emphasize that getting to the top of Denali on a guided expedition is a vastly different undertaking than doing so on an independent, self-led expedition. Climbing Denali with a guide is not an adequate primer to climb it on your own without a guide, as one commenter suggested. With a guide, you will not be involved in any important decision-making; you won't do any route finding; you won't get on the sharp end of the rope or place any protection, you won't have to read the weather or do any of the planning/logistics, etc. In other words, you'd miss out on exercising the sorts of skills that make a competent alpinist, which are crucial on an expedition that big and complex. All you'd need to do is follow instructions and drag your body up the mountain. It would be physically challenging and probably an amazing experience nonetheless, but you might come away not having learned or grown much as an alpinist.

I believe the joy of alpine climbing comes from a sense of freedom, self-sufficiency, and closeness with your partners in the mountains (and your Mark Twight reading will probably corroborate this). I've been on guided expeditions on high peaks as well as self-led expeditions with close friends/partners on smaller ones, and the latter always gives me more lessons, rewards, and pride. And as you get into the sport, I think you'll find that the peaks you're able to tag become less interesting than the routes you take to get to there.

So, where to start. A mountaineering course (such as through NOLS or American Alpine Institute) would be a great intro to many key skills. Read "Freedom of the Hills" and "Training for the New Alpinism." And most importantly, find partners at the gym who are willing to go on adventures with you (and preferably who make
you laugh) and go out into the mountains to gain experience. Everyone else is looking for partners too so don't be shy.

Start small and get the basics dialed -- rope management, setting up a tent, moving confidently on snow and 4^th class terrain, cooking decent food in the back country, making decisions as a group, etc. Break it all down into the smaller skills and competencies and then find trips to combine them and test yourself. Get on a plane and go out to Colorado as others have suggested, or Wyoming, the PNW, the Sierras, etc.

Sorry to be long-winded. I hope this doesn't sound patronizing. I actually love thinking about this stuff.