What does فِي سَبِيلِ اللَّهِ mean?

I read a the hadith in Sunan Ibn Majah 1717 : > It was narrated from Abu Sa’eed Al-Khudri that the Messenger of Allah > (ﷺ) said: “Whoever fasts one day in the cause of Allah, Allah will > keep the Fire away from his face the distance of seventy autumns > (years) for that day.” Here the word used is فِي سَبِيلِ اللَّهِ In Jami` at-Tirmidhi 1632 the word is used like bellow: > Narrated Yazid bin Abu Maryam: > > "Abayah bin Rifa'ah bin Rafi' met me while I was walking to the Friday > prayer. He said: 'Have glad tidings, for indeed these footsteps of > yours are in the cause of Allah. I heard Abu 'Abs say: "The Messenger > of Allah (ﷺ) said, 'Whoever get his two feet dusty in the path of > Allah, then they are prohibited for the Fire.'" However, by فِي سَبِيلِ اللَّهِ I understand fight so that Allah's Word (Islam) should be superior (Sahih al-Bukhari 7458 ). My understanding is, Cause of Allah is not the same as Sake of Allah (Ikhlas). So how, "walking to the Friday prayer" or "fasts" -- is in the Cause of Allah (فِي سَبِيلِ اللَّهِ).