Yes the streets are walkable. We walked from Cadorna to Hotel London which is on via Rovello and about 1/2 way between Cadorna and the Duomo which is the station closer to Spadari. It did seem somewhat long mainly because we were carrying our luggage and we were unfamiliar with the area. I remember being a bit confused when we walked up to the street level but this has happened before in other subways/train stations. It is tough to orient yourself. If I remember correctly via Dante isn't too busy.
With respect to safety, the streets are certainly walkable, and some are pedestrian zones only (like the via Dante) but I would estimate the walk to be at least 35 to 40 minutes, not taking into account the possibility of getting lost.
My concern would be that it might be quite hot. By the time you walk from your plane to baggage pick up to the Malpensa Xpress, you will have done quite a bit of walking as is. A taxi to your hotel from Cadorna would be inexpensive, and then you could walk to your heart's content without your backpacks.
It's only a minor consideration, but the route between Cadorna and the Duomo goes through some relatively posh areas of Milano. I never care two figs if I look like a wrinkly, sleep-deprived tourist while everybody else looks like a crisp, successful professional, but you are likely to be the only people hauling backpacks in that area if it matters to you.
Cadorna station is in the very heart of the Milano center. Most of the streets you will be walking will be pedestrian only, you will be walking near the Castle, across Largo Cairoli, past the American Bookshop and all the way up via Dante, across piazza Cordusio, on the right along via Orefici to the inersection with via Torino, and on the right to Via Speronari. It's an easy, shortish walk uless you are burdened with luggage. other wise, I suggest taking the subway from Cadorna to Duomo, which has an exit right on via Torino.