It is nearly half past five we cannot reach town before dark.
Although this is generally considered a stylistic error, it is occasionally used in literature and may be used as a rhetorical device.
Run-on sentences occur when two or more independent clauses are joined without using a coordinating conjunction (i.e., for, and, nor, but, or, yet, so) or correct punctuation (i.e., semicolons, dashes, or periods). 
A run-on sentence can be as short as four words -- for instance, I drive she walks. In this case there are two subjects paired with two intransitive verbs. However, with correct punctuation, a writer can assemble multiple independent clauses in a single sentence; a properly constructed sentence can be extended indefinitely.
There are several ways to correct a run-on sentence.
- Insert a semicolon or dash:
- It is nearly half past five; we cannot reach town before dark.
- It is nearly half past five—we cannot reach town before dark.
- Write the two clauses as two separate sentences (Note: this may disconnect related independent clauses and cause some of the meaning to be lost):
- It is nearly half past five. We cannot reach town before dark.
- Insert a coordinating conjunction with a comma:
- It is nearly half past five, so we cannot reach town before dark.
- It is nearly half past five, and we cannot reach town before dark.
- In British English, there would be no comma before the word and (i.e., It is nearly half past five and we cannot reach town before dark.)
- Make one clause dependent on the other:
- Because it is nearly half past five, we cannot reach town before dark.
- It is nearly half past five, which means we cannot reach town before dark.
Although the run-on sentence is considered grammatically incorrect, there are numerous examples of its use in literature.
- The short story ""Repent, Harlequin!" Said the Ticktockman" is known for its use of improper grammar, one example of which is a paragraph about jelly beans composed almost entirely of run-on sentences.
- James Joyce's novel Ulysses comprises streams of consciousness that often take the form of long and unpunctuated run-on sentences, particularly the final chapter, "Penelope".
- Examples adapted from the online, public-domain 1918 edition of The Elements of Style.
- Berry, Chris; Brizee, Allen (2006-08-31). "Run-ons – Comma Splices – Fused Sentences". Retrieved 2008-01-24.
- "Run-on Sentences, Comma Splices". Retrieved 2008-01-24.
- Hairston, Maxine; Ruszkiewicz, John J.; Friend, Christy (1998). "The Scott, Foresman Handbook for Writers" (5th ed.). New York: Longman: 509.