ほっそい道にほとんどスピードを落とさず、走りこんだ。「近道なんですよ」「ってここ、あぜ道～～～っ！」

、。「」「、！」

Without really slowing down, he charged onto the narrow road. 'It's a shortcut!' 'Wait, this is a rice-paddy path!!'

Literal

Narrow road [onto-に] hardly speed [obj-を] drop-[without-ず], dashed-in. 'Shortcut [copula-なん] [polite-です] [emph-よ].' '[quot-って] here, paddy-path ~~~~ [emph-っ]!'

ほっそい is a colloquial deformation of 細い 'narrow' — the initial syllable is doubled and the vowel is lengthened for emphatic effect, a casual-speech pattern also seen in あっつい (熱い), でっかい (大きい). 走りこむ is a compound verb 走る + こむ 'to run into / dash into.' 落とさず 'without dropping (speed)' is the classical negative conjunctive form ず (without final に). The quoted って at the start of the reply is a casual marker of interrupt/incredulity, roughly 'wait, what — ?'