Marx's Kapital was first published in 1867, but it wasn't until 1917, fifty years later, that the first communist state came into existence. This means that Marxist thinkers had fifty years to iron out the details and produce a relatively consistent package of ideas to which most Marxists adhere.
In contrast, the first stirrings of fascist thought showed up during WW1 with thinkers like Gabriele D'Annunzio and Enrico Corradini, while Mussolini's March on Rome took place in 1922. This means that fascism as an ideology had less than ten years to develop from a vague intellectual movement to a coherent, practical political ideology.
Did fascists actually know what they were fighting for, or was it more of an emotional outburst against failure in WW1 and the rise of the left?
You can pretty much state that fascism developed with Mussolini and his companions. Certain elements of fascism were present before hand. Social-Darwinist thought and eugenics (with a racist element and scientific racism) have long roots from the 19th century on. The Völkische Bewegung in Germany, is an important predecessor for the Nazis thoughts about the Volk, racial purity and antisemitism.
Corporatism in its essence was not something completely new. Pope Pius XI basically advocated it in Quadragesimo anno, to combat socialist revolutionary tendencies while also acknowledging that the conditions of the working man aren't acceptable.
Voluntarism was not something that specific to Fascism. Communists, especially of the Soviet style, were very much voluntarist too. This became even more apparent with Stalin, but was an element before him.
Anti-Communism in its ideological variant was very common along the middle class. This is coupled with a suspiciousness of Capitalism. This is how in NS Propaganda Jews can be Capitalist and Communist at the same time and also holds true for Italian fascism. Capitalism and Communism both destroy the nation state and therefore the life blood of the Volk.
Fascism was the amalgation of all these components into a somewhat coherent ideology. But for fascism ideological intricacies were never that important (as say in 19th century Socialism), it was about taking action, acting and doing. Fascism developed a very extreme and strong dynamic very fast. From its conception and fusion into a political party to taking power in Italy and spreading into Europe within a decade is without a parallell I think.
A thoughtful response was posted on, of all places, 4chan. I will post it in full:
It never did, even after Mussolini's march, no coherent form of fascism developed, if you look at the fascist parties of Europe
The difference between Hitler and Mussolini's fascism is the obvious comparison, but there are the other parties of Europe who never came to power but were organised and existed too
The British Union of Fascists also from socialist roots like Mussolini advocated a corporatist state, with workers voting based on their professions, electing representatives of that profession to represent them, wanting to create a parliament of experts, believers workers should take 50% ownership of any company over a certain size, and that the major state industries would be placed under government control, whilst socially followed the original Italian racial theories based on culture, while taking a reconciliary approach to religion, supporting neither Protestantism of Catholicism
The Resist party in Belgium closely followed Nazi lines in both racial theory and economics, however was strictly royalist and supporting the Union of Walloons and the Flemish, while following a devoutly Catholic progam
Fascism seemed to develop into two ideologies, German and Italian. The Italian branch generally resonated with those who came from the left of politics dissatisfied with the political elite and the old guard of politics, putting the economy national prosperity first. The German model was mostly followed by parties that originated in the right like the NDASP and the Rexists, putting nationalism and racial purity as their priorities
Due to the different developments of the the fascist parties in the only countries where they came to power, it left the ideology permanently split
This is why I'd argue two forms of fascism developed in the end
'German' fascism developed by men of the right disillusioned by the war or the aftermath, Adolf Hitler and Leon degrelle, focussing on the nationalistic elements and supporting a form a mutated state capitalism
'Italian' fascism developed by former socialists disillusioned by the war Benito Mussolini, and Oswald Mosley, focussing on the economy or representation of the people, supporting a form of National syndicalism