Boots of mobility are a poor choice because you will be 'in combat' every time you attack someone and the speed buff will go away. If you really want to be super-fast, you could use boots of swiftness, however berserker's greaves are standard on almost all ADC champs including Ashe. If you're having trouble chasing or kiting, get the furor enchantment on them for extra speed whenever you're fighting.
zz'rot portal -does- provide both armor and MR, but Guardian Angel is a stronger defensive item for the same thing. Mostly, though, you should figure out if they enemy team deals more physical or magical damage, and itemize accordingly. Randuin's vs. an all-AD team, Banshee's Veil against a heavy AP team, Mercurial Scimitar against a team that has CC that you need the active to remove. Guardian Angel is good against many assassins (especially those with a reset-on-kill mechanic), but the passive is on a huge cooldown. You might even take thornmail against multiple autoattackers, especially when people put an ADC both mid and bot.
The remainder of your build is not exactly terrible but not exactly optimum. Runaan's feels good on Ashe, but you already have volley to apply an AoE slow and you sacrifice a lot of single-target damage by not choosing statikk shiv or phantom dancer instead. Statikk in turn gives you more wave clear (and a little AoE damage on enemy champions) in exchange for a bit less single target damage compared to Phantom Dancer. (Because these two attack speed items come with critical strike chance, which scales just as strong on Ashe as any ADC, just a little differently.)
Zephyr is a good second attack speed item, but armor pen (last whisper, black cleaver) are going to help more... one attack speed item (+berserkers greaves) is usually enough. Excessive attack speed goes to waste anyway, you can only get an attack speed of 2.5 (attacks per second). That's a hard cap, any attack speed beyond that is simply wasted gold. I haven't run the numbers but with blade of the ruined king -and- two of the highest attack speed items in the game, I'm sure you're over the cap.
Check out guides for champions you play ... lolpro, lolking, mobafire ... there are a ton of sites that provide build guides to get you started, although experimenting and learning to adapt is also important. Just google 'league ashe guide' (or substituting whatever champion) and you should find some sites with useful advice.
& my typical build with her is:
Runaan's Hurricane (Ranged Only)
Infinity Edge
Zephyr 
Boots of Mobility 
with Captain Enchantment 
Zz'Rot Portal 
Blade of the Ruined King 
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