This way to play is only for ofensive tatics. Why mana runes, it´s easy the power of singed is his mana, because he change mana for hp, if you want live you can take mana. The most important thing that you must do is Fling for your team can kill easy one enemy hero. The boots of mobility are very good for help your friends or push one line, don´t worry about with your speed because you have runes, and habilities, your ultimate and talents, make you run more fast tha the other heros. This is the reazon for that you must buy boots of movility. But you can take other if do you prefer. I take Exhaust because you can make easy the first kill whit your parther in the same line.
You are faster than your enemyes and this is the most important and the real dmg from singed.
You must make Fling and then Exhaust, and probably he will death if your parther can make a little bit of damage. I think thast is not good idea make a Singed ap, because is more expensive make damage with poison, and more cheap the sun fire and you earn hp and armour.

The order of the objects you can change in the game for example if they have a lot of magic dmg you can take early the force of nature, with 8% of movement speed very intresting, and magic resit. If they have a lot of fisical dmg you must buy armour.

Important: if you are the first time that you take Singed you must buy a blue crystal and 2 potions of hp instead of the read gem, but try to make fast the catalyst. If you go a little bit late don´t worry buy then boots of level 1, and then finish catalyst and then upgrade boots.It´s important the catalyst because you earn hp and mana, and heal you, to stay more time farming in the line. The ulti is very strong you can kill some one with this but you must use your poison, always run in the same way than the enemys and a litle bit faster. And the poison make all the work.

Prove this bouild and beleve in Singed.

Important force of nature, because your movement speed increes, hp regen and magic resit. Is important take fast but if you are a good team helper this is not a problem. The next item is dificult because if you make dmg and can kill heroes easily take the next item, because you make more dmg and help your team with the pasive skill.