Looking this route on the map it feel like a long stretch but is actually relatively easy.
Leaving the mountains on your back from Como to Milan is a gravel path along the Adda. This river is nice and calm, you are not gonna find many people along the way, make sure to have enough water with you as there are fountains but not as many as you might need.
You keep following the river going south till you reach a village called Vaprio d’Adda, there you can leave the river to a canal that was specifically made to take some of the Adda’s water and bring it to Milan.
This is a paved path all the way to Milan. It’ll get busier and busier while approaching the city obviously but it also run on the other side of the canal so there’s space for everyone.
Without even noticing you arrive in Milan, city center, from here be careful of not loosing the cycle path that goes to the Navigli (don’t do my mistake which took 45 minutes of wondering in Milan city centre). The real fun begins now: if you are on the Navigli, south of Milan, you have a straight line (another canal) going down in a 47 km well paved road to Pavia.